Semiconductor lasers have been widely used as excitation light sources of solid-state lasers since these have advantages such as high efficiency, long life, and small size. Particularly, semiconductor laser array stacks have been drawing attention as semiconductor lasers that can obtain a high output. In the semiconductor laser array stack, a plurality of semiconductor laser arrays are laminated in the vertical direction. The semiconductor laser array has a plurality of emitters arrayed in the horizontal direction. In an example of the semiconductor laser array, some single-mode active layers are arrayed in a stripe form.
Since a semiconductor laser that carries out a continuous oscillation or a pulsed oscillation with a high duty ratio has a great heat value, it is necessary for a semiconductor laser array stack to have arranged a radiator plate or a heat sink between the semiconductor laser arrays. Since the radiator plate or heat sink normally has a thickness on the order of 1 to 2 mm, the semiconductor laser arrays also have an interval therebetween on the order of 1 to 2 mm. As a result, a parallel light flux group emitted from these semiconductor laser arrays forms a striped pattern within a plane vertical to optical axes of the light fluxes. Since the light flux group is arrayed at intervals corresponding to the intervals of the semiconductor laser arrays, optical density of the light flux group tends to be low.
Japanese Published Unexamined Patent Application No. Hei. 4-78180 discloses an apparatus that increases the density of a striped light flux group. By synthesizing laser light fluxes outputted by two semiconductor laser array stacks by use of a polarizing beam splitter, the apparatus generates a laser light flux group with a higher density than that of a laser light flux group emitted from a single semiconductor laser array stack.
The apparatus employs a polarizing beam splitter in order to synthesize laser light flux groups outputted by the two semiconductor laser array stacks. Therefore, by making light flux groups outputted by the respective semiconductor laser stacks incident into the polarizing beam splitter while differentiating polarizing directions thereof by 90 degrees from each other, a light flux group having a higher density can be efficiently generated. However, the apparatus cannot generate a light flux group with a higher density from laser light flux groups outputted by three or more semiconductor laser array stacks.
Moreover, in the laser light flux group outputted by the two semiconductor laser array stacks and synthesized by use of the polarizing beam splitter, fluxes having two different polarizing directions are mixed. Therefore, even when the light flux group is irradiated onto a solid-state laser medium having polarization dependency in light absorption, represented by Nd:YLF and Nd:YVO4, excitation efficiency cannot be sufficiently improved.
Another apparatus for improving the density of a striped light flux group is disclosed in a German patent specification DE 197 51 716 C2. The apparatus synthesizes light flux groups outputted by three laser array stacks by use of an optical device having a structure in which a first prism plate, an optically transparent plate, and a second prism plate are stacked up in order.